sd_event_add_memory_pressure, sd_event_source_set_memory_pressure_type, sd_event_source_set_memory_pressure_period, sd_event_trim_memory — Add and configure an event source run as result of memory pressure
typedef struct sd_event_source sd_event_source;
|const char *type|
sd_event_add_memory_pressure() adds a new event source that is triggered
whenever memory pressure is seen. This functionality is built around the Linux kernel's Pressure Stall Information (PSI) logic.
Expects an event loop object as first parameter, and returns the allocated event source object in
the second parameter, on success. The
handler parameter is a function to call when
memory pressure is seen, or
NULL. The handler function will be passed the
userdata pointer, which may be chosen freely by the caller. The handler may return
negative to signal an error (see below), other return values are ignored. If
NULL, a default handler that compacts allocation
caches maintained by
libsystemd as well as glibc (via malloc_trim(3))
will be used.
To destroy an event source object use
but note that the event source is only removed from the event loop when all references to the event
source are dropped. To make sure an event source does not fire anymore, even if it is still referenced,
disable the event source using
If the second parameter of
NULL no reference to the event source object is returned. In this case the event
source is considered "floating", and will be destroyed implicitly when the event loop itself is
The event source will fire according to the following logic:
variables are set at the time the event source is established, it will watch the file, FIFO or AF_UNIX
socket specified via
$MEMORY_PRESSURE_WATCH (which must contain an absolute path
POLLPRI (in case it is a regular file) or
events (otherwise). After opening the inode, it will write the (decoded) Base64 data provided via
$MEMORY_PRESSURE_WRITE into it before it starts polling on it (the variable may be
unset in which case this is skipped). Typically, if used,
will contain a path such as
/proc/pressure/memory or a path to a specific
memory.pressure file in the control group file system
If these environment variables are not set, the local PSI interface file
memory.pressure of the control group the invoking process is running in is
If that file does not exist, the system-wide PSI interface file
/proc/pressure/memory is watched instead.
Or in other words: preferably any explicit configuration passed in by an invoking service manager (or similar) is used as notification source, before falling back to local notifications of the service, and finally to global notifications of the system.
Well-behaving services and applications are recommended to react to memory pressure events by executing one or more of the following operations, in order to ensure optimal behaviour even on loaded and resource-constrained systems:
Release allocation caches such as
malloc_trim() or similar, both
implemented in the libraries consumed by the program and in private allocation caches of the program
Release any other form of in-memory caches that can easily be recovered if needed (e.g. browser caches).
Terminate idle worker threads or processes, or similar.
Even exit entirely from the program if it is idle and can be automatically started when needed (for example via socket or bus activation).
Any of the suggested operations should help easing memory pressure situations and allowing the system to make progress by reclaiming the memory for other purposes.
This event source typically fires on memory pressure stalls, i.e. when operational latency above a configured threshold already has been seen. This should be taken into consideration when discussing whether later latency to re-aquire any released resources is acceptable: it's usually more important to think of the latencies that already happened than those coming up in future.
sd_event_source_set_memory_pressure_period() functions can be used to fine-tune the
PSI parameters for pressure notifications. The former takes either "
full" as second parameter, the latter takes threshold and period times in microseconds
as parameters. For details about these three parameters see the PSI documentation. Note that these two
calls must be invoked immediately after allocating the event source, as they must be configured before
polling begins. Also note that these calls will fail if memory pressure parameterization has been passed
in via the
environment variables (or in other words: configuration supplied by a service manager wins over internal
sd_event_trim_memory() function releases various internal allocation
caches maintained by
libsystemd and then invokes glibc's malloc_trim(3). This
makes the operation executed when the handler function parameter of
sd_event_add_memory_pressure is passed as
accessible for invocation at any time (see above). This function will log a structured log message at
LOG_DEBUG level (with message ID f9b0be465ad540d0850ad32172d57c21) about the memory
For further details see Memory Pressure Handling in systemd.
On success, these functions return 0 or a positive integer. On failure, they return a negative errno-style error code.
Returned errors may indicate the following problems:
Not enough memory to allocate an object.
An invalid argument has been passed.
$MEMORY_PRESSURE_WATCH variable has been set to the literal
/dev/null, in order to explicitly disable memory pressure
$MEMORY_PRESSURE_WATCH variable has been set to an invalid
string, for example a relative rather than an absolute path.
$MEMORY_PRESSURE_WATCH variable points to a regular file
outside of the procfs or cgroupfs file systems.
No configuration via
$MEMORY_PRESSURE_WATCH has been specified
and the local kernel does not support the PSI interface.
This is returned by
sd_event_source_set_memory_pressure_period() if invoked on event sources
at a time later than immediately after allocating them.
The event loop is already terminated.
The event loop has been created in a different process, library or module instance.
The passed event source is not a signal event source.
Functions described here are available as a shared
library, which can be compiled against and linked to with the
The code described here uses
which is declared to be not multi-thread-safe. This means that the code calling the functions described
here must not call
from a parallel thread. It is recommended to only do calls to
from an early phase of the program when no other threads have been started.